Different vehicle concepts, where the internal combustion engine of the vehicles can be shut down during suitable operating states, available for the purpose of reducing the consumption and noxious substance emissions of the vehicles are known from the practice. They are configured with conventional internal combustion engines, such as gasoline-powered or diesel-powered internal combustion engines. These functions are called engine start/stop functions and are activated or de-activated, depending upon the operating states of the various vehicle components. They shut down an internal combustion engine even during short vehicle standstills when they are activated and start it up again when there is a demand for a startup of the internal combustion engine.
In order to prevent affecting the driving operation with an engine start/stop function, a short startup process of the internal combustion engine and an immediate build up of power transmission in a transmission of the vehicle take place when the driver of the vehicle demands to continue driving, especially when driving into high traffic and streets with the right of way. In conventionally designed automatic transmissions or automated manual transmissions, which are configured with wet-running disk clutches, the clutches are supplied with the required startup pressure via a transmission pump essentially only when the internal combustion engine is running. The buildup of power transmission must first overcome a slack of the clutches to be activated and the clutches to be activated are, in addition, to be completely engaged by raising the startup pressure, according to predetermined activation guidelines, Compensation for the slack of a clutch, as well as its activation in the power flow of a transmission, is achieved by directing a specific hydraulic fluid volume to flow into a piston chamber of the hydraulically controllable and activatable clutch, which can be accomplished by the transmission pump driven by the started internal combustion engine.
If several shifting elements or a transmission mechanism are disengaged before the vehicle is driven again, due to the fact that the engine is shut down and are then engaged to again drive the vehicle, the time, between the start of the startup procedure of the internal combustion engine and the point in time at which the powerflow has been completely reestablished in the transmission, is extended under certain circumstances to such an extent that a vehicle is provided with an engine start/stop function cannot be operated within the sought range of time.
In order to still be able to operate vehicles having conventional transmission mechanisms with an implemented engine start/stop function in the desired manner, aside from the main transmission pump of the known vehicles, a further hydraulic pump operated by way of an electric motor, is also provided, whose delivery volume is independent from the rotational speed of the internal combustion engine and which at least generates a pressure in the hydraulic system when there is no pressure supply, via the main transmission pump of the transmission mechanism, which generally compensates for the slack of the clutches, activated in the transmission in order to produce the power flow in the transmission.
The tests have shown that a driver of a vehicle is particularly aware of the time period between a point of releasing a brake pedal and the point in time at which an input torque builds up in the output of the vehicle, for which reason the customer acceptance of these start/stop systems, during the startup procedure of the internal combustion engine, depends essentially on the amount of reaction time the power train takes to establish an amount of output torque, necessary for vehicle acceleration, at the output. An operating state characteristic, that is critical with regard to the evaluation on the driver side of the spontaneity of an engine start/stop system, is present upon a full-load startup with simultaneous actuation of the brake pedal following the previous engine stop. During an operating state characteristic such as this, the period, starting with the actuation of the accelerator pedal by the driver up to torque buildup at the output, is experienced as particularly bothersome for the driver.
In power trains, having a combination of an automatic transmission and a startup system with a hydrodynamic torque converter or a non-positive startup clutch, the flow of power within the transmission, during the shutdown phases of the internal combustion engine, is generally ensured, via a previously described supplementary pump, or the power flow is interrupted in the area of the automatic transmission when a selector lever is in the neutral or park position. In the last mentioned operating state of the automatic transmission, the shifting elements of the automatic transmission, which are configured as shifting clutches or shifting brakes and can be hydraulically controlled, are shifted without pressure and are in an essentially disengaged state.
If a demand is made for starting the internal combustion engine output torque, at the output of the vehicle, occurs only when the rotational speed of the internal combustion engine has reached a defined value, at which torque is forwarded in the direction of the transmission input of the automatic transmission connected downstream of the converter, because of a characteristic converter identification of a torque converter of a starter unit of a power train.
In one embodiment of a power train with an automatic transmission and a starter unit with a startup clutch, a hydraulically controllable startup clutch is engaged without pressure without a hydraulic supplementary pump when the engine is shut down. This results from the fact that the main transmission pump is usually driven by the internal combustion engine and for this reason a corresponding startup pressure is only made available to the startup clutch when the internal combustion engine reaches a corresponding rotational speed. The main transmission pump has merely a limited delivery rate during the startup phase of the internal combustion engine, during which the latter has low rotational speeds. For this reason, the time period between the point in time, starting at which the driver can expect a torque at the output exceeds the time point starting where the startup clutch has an available transfer capacity with a desired limit value where a torque can be fed in the direction of the output, via the startup clutch.
It is, therefore, an object of the invention to make a process for operating a power train of a vehicle available during an activated engine start/stop function for an internal combustion engine, where the spontaneity of vehicles, designed with engine start/stop functions, can be easily and economically improved.